Effects of high pressure and high-low pressure EGR strategies on the knock tendency, combustion, and performance of a stoichiometric operation natural gas engine

Abstract

EGR strategies play an important role in improving the combustion and performance of a stoichiometric operation natural gas engine. Based on the numerical simulation and experimental validation, the effects of two EGR strategies, namely high pressure (HP) EGR and high-low pressure (HLP) EGR on the combustion behavior and knock tendency of a natural gas engine were investigated. The optimal EGR rates and spark timings under different full load conditions were obtained, and the reason of the HLP EGR strategy in improving fuel economy was quantitatively evaluated. The simulation results showed that compared with the HP EGR strategy, the optimal EGR rates are reduced and the optimal spark timings are delayed for the HLP EGR strategy. This is due to that the lower boost pressure makes it unnecessary for the HLP EGR strategy to adopt higher EGR rates to suppress knock. Meanwhile, the HLP EGR strategy shortens the flame development period and rapid combustion duration, makes the center of heat release rate curve closer to TDC, increases the peak values of in-cylinder pressure and heat release rate, and significantly reduces the pumping losses. As a result, the HLP EGR strategy increases the indicated thermal efficiency by 3.4% − 7.3%. The dyno experiment of the natural gas engine with the optimized EGR rates and spark timings showed that compared with the HP EGR strategy, the HLP EGR strategy reduces the specific gas consumption by 5.82% − 8.83%. The engine energy balance analysis showed that the in-cylinder heat-transfer losses and exhaust energy for the HLP EGR strategy are reduced, contributing to the fuel saving rates by 2.18% − 2.47% and 4.02% − 6.91%, respectively. While the friction losses are increased, resulting in the reduction in the fuel saving rates by −0.38% to −0.55%. Therefore, the reduction of exhaust energy is the most important fuel saving factor for the HLP EGR strategy.